The present invention relates generally to a system and method for detecting the liquid level and storage amount of a liquid stored in a vessel using an ultrasonic sensor by providing a detector outside a pressurized vessel to emit ultrasonic waves from the outside of the vessel to the inside thereof.
A typical ultrasonic sensor is designed to emit an ultrasonic pulse toward an object to be detected (the surface of a liquid) to receive a reflected wave, which is reflected on the object, to measure a period of time from the ultrasonic pulse emitting time to the reflected wave receiving time to detect the object (the liquid level of the liquid). Therefore, a material for inhibiting the smooth propagation of ultrasonic waves is not preferably provided between a detector for transmitting/receiving the ultrasonic pulse and the object to be detected. Thus, there is proposed a liquid measuring system wherein piezoelectric active means of a piezoelectric element for emitting a sound wave is provided on the bottom of a vessel, such as a fuel tank of an automotive vehicle and wherein when the sound wave emitted from the bottom of the vessel is reflected on the liquid level to reach the bottom again, the sound wave is received to measure a period of time from the emission time to the receiving time to detect the distance between the bottom and the liquid level (e.g., see International Publication No. WO98/04889).
However, when a liquid serving as an object to be detected, such as liquefied petroleum gas (LPG) or gasoline, is stored in a pressurized pressure vessel, so that it is difficult to provide a detector in the vessel, the detector is unavoidably provided outside of the vessel to transmit/receive the ultrasonic pulse between the detector and the object via the thick wall portion of the vessel. In particular, the main current of, e.g., a level sensor for detecting a liquid level in an LPG tank, uses mechanical means. If an ultrasonic sensor is used as the level sensor, a detector is mounted on the outside bottom face of the tank since the LPG tank is a pressure vessel and since it is difficult to provide the detector in the vessel.
In such a construction, the ultrasonic wave emitted from the detector passes through the thick bottom wall portion of the tank to reach the liquid level via the liquid to be reflected on the liquid level to pass through the thick bottom wall portion of the tank via the liquid again to be received by the detector. Therefore, in order to precisely detect the liquid level, the transmittance of the ultrasonic wave in the thick wall portion of the bottom plate of the tank (pressure vessel) must be high.
However, in order to enhance the transmittance of the ultrasonic wave in the thick wall portion of the bottom plate of the tank, the oscillation frequency of the ultrasonic wave emitted from the detector must be optimally set in accordance with the wall thickness and material of the tank. That is, with respect to the relationship between the transmittance and the wall thickness of the tank, it is known that the transmittance is good when the wall thickness is integer times as large as (xc2xd)xcex or (xc2xc)xcex assuming that the wavelength of an ultrasonic wave is xcex. Therefore, assuming that the wall thickness is t, the oscillation frequency is f, the sound speed is c, and an integer is n, then xcex=c/f, so that the wall thickness t is expressed by the following formula (1) or (2). Furthermore, whether the wall thickness t is expressed by the formula (1) or (2) is determined by whether the piezoelectric element has a vibrating plane(s) on one side or both sides.
t=nxc2x7(xcex/2)=nxc2x7c/2fxe2x80x83xe2x80x83(1)
t=nxc2x7(xcex/4)=nxc2x7c/4fxe2x80x83xe2x80x83(2)
With respect to the relationship between the transmittance and the material of the tank, it is known that the transmittance is good when the value of the resonance frequency, which is a characteristic value peculiar to the material, is close to the value of the operating frequency of the piezoelectric element of the detector. Thus, conventionally, when the detector is mounted on the outside bottom face of the LPG tank, ultrasonic sensor manufacturers previously examine the wall thickness and material of the tank to select sensors having the optimum operating frequency from existing standard products, or new product ultrasonic sensors if there are no sensors having the optimum operating frequency in the existing standard products.
However, the wall thickness and material of the tank vary in accordance with working environment or other conditions, so that there are some cases where an operating frequency having been applied to a certain tank can not be applied to another tank.
In such cases, the sensor manufacturers must select sensors having a desired operating frequency from the existing standard products again, or new product ultrasonic sensors if there are no sensors having the desired operating frequency in the existing standard products. That is, conventional ultrasonic sensors are exclusive goods having an operating frequency according to a specific working environment, and do not have flexibility, so that the sensors can not be used if the wall thickness or material of a vessel is changed. An example of such a conventional ultrasonic sensor is proposed in GB patent No. 2284053A.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a detection system having a general ultrasonic sensor applicable if the thickness and/or material of a vessel for storing therein a liquid serving as an object to be detected is changed, and an object detecting method using the above described ultrasonic sensor.
In order to accomplish the aforementioned and other objects, according to a first aspect of the present invention, an object detecting system using an ultrasonic sensor, comprises: a detector mounted on an outside face of a vessel; and a detection operation control circuit for causing the detector to emit an ultrasonic wave toward an object to be detected in the vessel and to receive a reflected wave from the object, and for controlling an object detecting operation an the basis of a measured period of time between the ultrasonic wave emitting time and the reflected wave receiving time, wherein the detection operation control circuit has operating frequency setting means for causing the detector to emit the ultrasonic wave a plurality of times while previously varying a frequency before the object detecting operation is carried out, for inputting an emission waveform or a reflected waveform at that time, for detecting a resonance frequency of the system comprising the detector and the vessel on the basis of an analysis of the inputted waveform, and for setting the detected resonance frequency as an operating frequency.
In the object detecting system according to the first aspect of the present invention, the detector may emit the ultrasonic wave in response to of the input of a rectangular pulse signal. In addition, the operating frequency setting means may input the emission waveform or reflected waveform, which is used for the analysis, via a band pass filter so as to prevent a false detection of the resonance frequency due to a high harmonic oscillation. Moreover, the vessel may be used for storing therein a liquid, and the operating frequency setting means may determine whether the analysis is carried out using the emission waveform or the reflected waveform, on the basis of the presence of the liquid in the vessel.
Moreover, the analysis of the emission waveform or reflected waveform carried out by the operating frequency setting means may include the selection of a waveform having the minimum attenuated degree of an attenuation waveform from the emission waveform and the reflected waveform, and the derivation of an operating frequency of the selected waveform. In addition, the analysis of the emission waveform or reflected waveform carried out by the operating frequency setting means may include the selection of a waveform having the minimum impedance including the detector and the vessel, and the derivation of an operating frequency of the selected waveform. In addition, the operating frequency setting means may shift each input timing from the last input timing by a predetermined period of time when a plurality of emission waveforms or reflected waveforms are inputted on the basis of the plurality of emissions of the ultrasonic wave. Moreover, the detector may have a piezoelectric element formed of a low Q material having a gentle resonance waveform (Q) so that an acutance of the resonance waveform decreases.
In order to accomplish the aforementioned and other objects, according to a second aspect of the present invention, there is provided an object detecting method using an ultrasonic sensor having a detector on an outside face of a vessel for causing the detector to emit an ultrasonic wave toward an object to be detected in the vessel and to receive a reflected wave from the object and for detecting the object on the basis of a measured period of time between the ultrasonic wave emitting time and the reflected wave receiving time, the object detecting method comprising the steps of: causing the detector to emit an ultrasonic wave a plurality of times while previously varying a frequency before the object is detected; inputting an emission waveform or a reflected waveform at that time; detecting a resonance frequency of the system comprising the detector and the vessel on the basis of an analysis of the inputted emission or reflected waveform; setting the detected resonance frequency as an operating frequency; and causing the detector to emit an ultrasonic wave toward the object to be detected in the vessel, on the basis of the set operating frequency.
As described above, according to the present invention, when an object is detected while a detector is mounted on the outside face of a vessel, the detector emit an ultrasonic wave a plurality of times while previously varying the frequency thereof. At this time, an emission or reflected waveform is inputted to detect a resonance frequency of the system comprising the detector and the vessel on the basis of the analysis of the inputted waveform to set the detected resonance frequency as an operating frequency. Therefore, it is possible to provide a general ultrasonic sensor, which can be used for any vessels having any wall thickness and material and which can be installed by only a user without the need of the cooperation with the sensor manufacturer.